High-speed high-efficiency fault-tolerant surface permanent magnet (PM) machines are used in a variety of applications including automotive and aerospace applications, and have been found particularly effective in safety critical applications such as fuel pumps in aerospace applications.
Conventional PM synchronous electric machines employ permanent magnets as the magnetic poles of a rotor, around which a stator is disposed. The stator has a plurality of teeth that face the rotor. Alternatively, the machine may be designed so that the rotor surrounds the stator. For high-speed operation, a retaining sleeve is usually wrapped around the magnets as needed to keep the magnets in place. The retaining sleeve may be shrink fit upon the magnets to ensure a non-slip fit. Usually the retaining sleeve is made of one whole metallic piece for structural integrity. When the coils formed on the stator are energized, a magnetic flux is induced by the current through the coils, creating electromagnetic forces between the stator and the rotor. These electromagnetic forces contain tangential and/or circumferential forces that cause the rotor to rotate.
In order to achieve inherent fault-tolerance in these surface PM machines, there has to be complete electromagnetic, thermal, and physical isolation between the coils of the various phases. This is achieved by using fractional-slot concentrated windings where each coil is wound around a single stator tooth and each stator slot is occupied by one side of a coil. Since slots formed between the teeth and the permanent magnets on the rotor are spaced from each other, the magnetic flux passing through a tooth will pass through the neighboring tooth in the next moment as the rotor rotates.
This winding configuration generates significant space harmonics when the windings are excited. These harmonics generate asynchronous rotating fields in the air gap that can generate significant losses in the rotor and hence significantly reduce the machine efficiency and exacerbate heat removal. Additionally, because the retaining ring is formed of a single metallic piece for mainly mechanical reasons, it provides an easy path for the eddy currents generated by the asynchronous rotating fields in the air gap generated by the concentrated winding configuration. This significantly aggravates the rotor losses and can reduce the machine efficiency to unacceptable limits and/or cause overheating that may lead to permanent demagnetization of the magnets. These harmonics may also lead to other problems including rotor dynamic imbalance.
In order to minimize the sleeve losses and to achieve acceptable efficiency, it has been proposed to use winding configurations such as the distributed lap or distributed wave windings that have less harmonic contents but at the same time have a strong coupling among the different coils and phases. However, these winding configurations result in compromising the fault-tolerance
Therefore, a need exists for a fault-tolerant, high-speed, high efficiency PM machine that operates with reduced eddy-current losses and improved operating efficiency.
Other features and advantages of the present invention will be apparent from the following more detailed description of a preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate by way of example, the principles of the invention.